Electrical contact materials and contacts and methods of making the same



Aug. 5, 1947. M. R. SWINEHART ELECTRICAL CONTACT MATERIALS AND CONTACTSAND METHODS OF MAKING THE SAME Filed March 8, 1944 MIXTURE OFA METALLICOXIDE AND AMETALLIC SALT.

CONDUCTIVITY.

Patented Aug. 5, i947 ELECTRICAL CONTACT MA'EMIALS AND CONTACTS ANDMETHODS OF MAKING THE SAME Merle a. Swinehart, Milwaukee, Wis, assignorto Cutler-Hammer, Inc., Milwaukee, Wis., a corporation oi. DelawareApplication March 8, 1944, Serial No. 525,606

30 Claims. i

Another object is to provide electrical contacts formed of powderedmixtures of materials of such composition that attainment of a highdegree of density thereof by sintering or heat treatment is facilitated.

Another object is to provide a powder composition electrical contactwherein formation of a film or layer of a material different from themain current carrying metal upon the operating surface of the contact isinsured when in use.

Another object is to provide for inclusion in the contact composition ofa relatively small percentage of an ingredient which acts to provide asurface film adapted to assist in preventing bonding or sticking of thecooperating contacts.

Another object of the invention is to provide a powder compositioncontact including a metallic salt whereby the length of life and otheroperating characteristics thereof are greatly improved. I

Another object is to provide an electrical contact having a novelform'and arrangement of at least one ingredient thereof with respect toanother whereby the desirable characteristics of each are accentuated.

Another and more specific object of the invention is to provide apowdered silver base contact embodying arelatively small quantity of asilver halide whereby the current controlling capacity and life of thecontact ar greatly increased.

The single figure of the accompanying drawin is a fragmentarysectionalview, highly magnified, of an electrical contact embodying myinvention.

Although I will herein describe that phase of my invention relating toparticle size of the oxide as specifically applied to the improvement ofelectrical contacts composed essentially of silver and cadmium oxide, itis to be understood that the invention inthis respect is applicable tothe improvement of substantially all types of composition contacts madeby the methods of powder metallurgy or otherwise.

Heretofore it has been proposed to use amor. phous cadmium oxide to coatand thus separate 2 the individual metallic ticles of such compositioncontacts to prevent the cooperating contacts from welding or stickingtogether during operation.

As the result of extensive experiments I have iound that it is notnecessary or desirable to separate the metallic particles throughout thebody of the contact to prevent welding or sticking; but that it is onlynecessary to provide a film of cadmium oxide upon the active surface ofat least one of the cooperating contacts.

In a contact produced in accordance with my invention such a surfacefilm is continuously produced by disintegration of relatively large anddense particles of cadmium oxide embedded in a matrix of fine silver. Asthe embedded particles are exposed at the surface of the contact bywearing down of the latter, such particles are broken up and'spread overthe surface of the contact both by mechanical action and arcing action.

As will be apparent to those skilled in the art, a contact of thecharacter herein disclosed has the advantage, over known contacts ofthis general type, of greatl increased electrical and thermalconductivities, which are due to the presence of many unbroken paths offine silver throughout the depth or thickness of the contact.Furthermore such inner structure or character of the contact materialfacilitates densification thereof, because of its inherently greaterresponse to sintering or heat treatment.

In producing a contact of the aforementioned character composedessentially of silver powder and cadmium oxide, I prefer to employMerck's precipitated silver powder and a commercial form of cadmiumoxide which is initially of such fineness as to pass through a 325 meshscreen. As set forth in Patent N 0. 2,307,668, granted Jan. 5, 1943, toI. W. Cox, Mercks precipitated silver powder is characterized by afibrous texture, extreme whiteness, and unusual ability to cold weldunder pressure to a tough solid of considerable tensile strength. Suchcadmium oxide is then densifled by pressing, or by pressing and heatsintering operations, and thereafter ground in such a manner, or to suchan extent, that at least fifty per cent of the powder which passesthrough a mesh screen will be retained upon a 300 mesh screen. Thepressure employed fordensifying the cadmium oxide may vary throughout awide range, as for example, from 40,000 pounds per square inch toupwards of 100,000 pounds per square inch In the particular examplesherein-' (silver, for instance) par- 3 pounds per square inch wasemployed to produce slugs of cadmium oxide having a diameter of one andone-eighth inches, said slugs being thensubjected to a temperature of1600 degrees F, for a period of one hour.

However, prior to application of the aforementioned densifying processto the cadmium oxide I prefer to add thereto and intimately mixtherewith a suitable metallic salt; as, for example, silver iodide,silver bromide, silver chloride, cadmium chloride or cadmium sulphate.

The addition of one or another of such metallic salts (or combinationsof two or more thereof) improves to a marked degree the anti-welding ornon-sticking characteristics of the particular contact, and therebysubstantially increases the range of currents which such contact willmake and break. Such metallic salts when thus employed insure a moreeven wear of the contacts, and consequently prolong the life-of thelatter.

The aforedescribed granular powder composed v; of cadmium oxide and ametallic salt is intimately mixed with a given quantity of silver powder(such as Merck's precipitated silver powder), and such mixture ofpowders is then cold molded at a pressure of about 40,000 pounds persquare inch to provide preforms of approximately threefourths of an inchin diameter. Thereafter said preforms are subjected to a temperature ofapproximately 1600 degrees F. for about twenty minutes. After such heattreatment said pref-orms are subjected to a molding pressure ofapproximately 100,000 pounds per square inch to produce the finishedcontacts.

In the single figure of the drawing the numeral 5 designates a fragment,on a greatly enlarged scale for purposes of illustration, of anelectrical contact embodying my invention. In thissectional view, thenumeral 6 designates the base or matrix material, which consists of ametal of good electrical conductivity, such as silver; and the numeral 1designates particles or granules of a mixture of a metallic oxide (suchas cadmium oxide) and a metallic salt (such as silver iodide). Thematerials in powder form are treated in the manner aforedescribed toproduce the final article.

The following examples of contacts produced in accordance with myinvention indicate the non-welding characteristics thereof, and showother advantages thereof over contacts of the priorart.

(a) A set of contacts, three-fourths of an inch in diameter, formedunder heavy pressure and heat sintered, composed of an intimate mixtureof 87.5 per cent of Mercks precipitated silver powder, 11.86 per cent ofcadmium oxide (of the character aforedescribed), and 0.64 per cent ofsilver iodide, will make and break 5000 amperes of a 30 volt directcurrent resistance load approximately one hundred times before stickingoccurs. On the other hand, the most promising contacts of the prior artfor such a purpose will fail at once on such a load, and such prior artcontacts will make and break only 2000 ampere of a 30 volt directcurrent resistance load for the same contact life; namely, one hundredoperations.

(1)) A set of contacts three-fourths of an inch in diameter, formedunder heavy pressure and heat sintered, composed of an intimate mixtureof 82 per cent of Mercks precipitated silver powder, 16.2 per cent ofcadmium oxide prepared as aforedescribed, and 1.8 per cent of cadmiumsulphate, will make and break 2600 amperes of a 250 volt alternatingcurrent, 60 cycle, 50 per cent lagging power factor load at least onehundred times; whereas no known contacts of the prior art are inanywis'e dependable for such service.

(0) A set of contacts three-fourths of an inch in diameter. formed underheavy pressure and heat sintered, composed of an intimate mixture of '75per cent of Merck's precipitated silver powder, 21.25 per cent ofcadmium oxide prepared as aforedescribed, and 3.75 per cent of silverchloride, will properly make and break 4000 amperes of a 260 voltalternating current, 60 cycle, 40 per cent lagging power factor load atleast ten times. Such a performance is not possible with any knowncontacts of the prior art. Substantially the same novel result may beobtained by substituting for the silver chloride a corresponding amountof silver bromide, the proportions of the other ingredients remainingthe same.

In the foregoing examples the given proportions of the ingredients areby weight. Although I prefer to employ Mercks precipitated silver powderwhere a silver base contact is to be produced, it is to be understoodthat other silver powders may in some cases be found entirely suitable,and less expensive. Moreover, it will be apparent to those skilled inthe art that the speciflc proportions of the various ingredients may bevaried within relatively wide limits without sacrificing the importantadvantages of my invention, according to the particular use contemplatedfor the contacts.

While I have herein disclosed my invention as specifically applied tothe improvement of socalled silver-base contacts produced from powderedmaterials, it is to be understood that it is in nowise limited to usewith powdered silver. Thus the main ingredient of the powder used informing the contact may consist of a suitable alloy of silver; orpowdered copper, nickel, gold, etc. may be employed.

Although I do not know the exact reasons for the beneficial action ofthe added metallic salts in preventing sticking of the contacts and intheir abilityto extinguish very heavy alternating current arcs, thefollowing novel characteristics thereof are indicated:

1. It appears that the volatilized metallic salt produces an ionizedatmosphere at the operating surface of at least one of the cooperatingcontacts. Such ionized atmosphere effects spreading of the are at thearc feet, which inherently results in lower current density and reducedheating of the contact surface or surfaces by the arc. Such decreasedheating by the arc involves a reduction of pin-point formations ofmolten silver upon the contact or contacts, which pinpoint formations,if numerous, might cause welding or sticking of the cooperating contactsurfaces upon subsequent operations.

2. The condensation of the metallic salt or salts as a thin film uponthe contact surface or surfaces is believed to act to prevent bonding orsticking of the silvers of the cooperating contact faces in subsequentoperations.

3. The aforementioned silver halides (silver iodide, bromide, orchloride) in addition to preventing sticking of the cooperating contactswhen in operation, likewise act to prevent the contact resistance fromincreasing throughout the life of the contact. In a large percentage ofmany life tests conducted upon such contacts by me it was found that thecontact resistance actually decreased as the respective testsprogressed. It is thought that such additional beneficial action of thesilver halides (in reducing contact resistance) may be due the presenceof additional silver upon the surface of the contact or materialscomprising from sixty per .cent to ninety-three per cent by weight of ametal of good electrical conductivity, a second ingredient consisting ofcadmium oxide, and a metallic salt of the group consisting of silveriodide, silver bromide, silver chloride, cadmium chloride and cadmiumsulphate, said contact having nonsticking and substantially constantcontact resistance characteristics when in use.

2. An electrical contact formed of powdered materials comprising a metalof good electrical conductivity, a second ingredient consisting ofcadmium oxide, and a metallic salt of the group consisting of silveriodide, silver bromide, silver chloride, cadmium chloride and cadmiumsulphate, said cadmium oxide being present in the amount of about sevento forty per cent of the entire mass, and said metallic salt beingpresent in the least useful amount up to about ten per cent of saidmass, said contact having nonsticking and substantially constant contactresistance characteristics when in use.

3. An electrical contact formed of powdered materials comprising a metalof good electrical conductivity, a second ingredient consisting ofcadmium oxide, and a metallic salt of the group consisting of silveriodide, silver bromide, silver chloride, cadmium chloride and cadmiumsulphate, said cadmium oxide being present in the amount of about sevento forty percent of the entire mass, said metallic salt being present inthe amount of one-tenth ofone per cent to about ten per cent of saidmass, and at least fifty per v cent of the amount of said cadmium oxideconsisting of particles of a size greater than that which will passthrough a 300 mesh screen.

silver powder of fibrous texture, from seven to forty per cent ofcadmium oxide, and from onetenth of one per cent to ten per cent of asilver halide, all of said cadmium oxide being in the form of denslfiedparticles which will pass through a. 100 mesh screen, and the majorportion of said particles being of a size greater than that which willpass through a 300 mesh screen.

8. A pressure molded and heat sintered contact composed of a mixture ofapproximately eighty-seven and one-half per cent by weight ofprecipitated silver powder of fibrous .texture, approximately eleven andeighty-six one-hundredths per cent of cadmium oxide, and approximatelysixty-four one-hundredths per cent of silver iodide.

9. A pressure molded and heat sintered contact composed of a mixture ofingredients in about the following proportions by weight: eighty-two percent of precipitated silver powder of fibrous texture, sixteen andtwo-tenths per cent of cadmium oxide, and one and eight-tenths per centof cadmium sulphate.

10. A pressure molded and heat sintered con- I tact composed of amixture of ingredients in about the following proportions by weight:seventy-five per cent of precipitated silver powder of fibrous texture,twenty-one and twenty-five one-hundredths per cent of cadmium oxide, and

4. A pressure molded and heat sintered contact composed of a mixture offrom sixty per cent to ninety-three per cent by weight of the entiremass of silver powder, a substantially smaller proportion of the massconsisting of cadmium oxide, and a still smaller proportion of the massconsisting of a metallic salt of the group consisting of silver iodide,silver bromide, silver chloride, cadmium chloride and cadmium sulphate.

5. A pressure molded and heat sinteredcontact composed of a mixture offrom sixty to ninety-three per cent by weight of precipitated silverpowder of fibrous texture, from seven to forty per cent of cadmiumoxide, and from onetenth of one per cent to ten per cent of a silverhalide.

6. A pressure molded and heat sintered contact composed of a mixture offrom sixty to ninety-three per cent by weight of precipitatedsilverpowder of fibrous texture, from seven to forty per cent of cadmiumoxide, and from onetenth of one per cent to ten per cent of a silverhalide, the major portion of said cadmium oxide being in the form ofdensifled particles of a size greater than that which will pass througha 300 mesh screen.

'7. A pressure molded and heat sintered contact composed of a mixture offrom sixty to ninety-three per cent by weight of precipitated three andseventy-five one-hundredths per cent of silver chloride.

11. A pressure molded and heat sintered con-- tact composed of a mixtureof ingredients in about the followin proportions by weight: seventy-fiveper cent of precipitated silver powder 'of fibrous texture, twenty-oneand twenty-five one-hundredths per cent of cadmium oxide, and three andseventy-five one-hundredths per cent of silver bromide.

12. A contact of the character set forth in claim 8, wherein the cadmiumoxide is in such form that all of its particles will pass through a 100mesh screen but less than half of the same will pass through a 300 meshscreen.

13. A contact of the character set forth in claim 9, wherein the cadmiumoxide is in such form that all particles thereof will pass through a 100mesh screen but less than half of the same will pass through a 300 meshscreen.

14. A contact of the character set forth in claim 10, wherein thecadmium oxide is in such form that all particles thereof will passthrough a 100 mesh screen but less than half of the same will passthrough a 300 meshscreen.

'15. A contact of the character set forth in clai 11, wherein thecadmium oxide is in such form that all particles thereof will passthrough a 100 mesh screen but less than half of the same will passthrough a 300 meshscreen.

16. An electrical contact formed of thoroughly mixed powdered materials,one of-said materials consisting of from sixty per cent to ninety-threeper cent by weight of a metal of good electrical conductivity, anotherof said materials comprising relatively large particles of cadmiumoxide, and said metal beingadapted to act as a matrix for said cadmiumoxide, whereby, the electrical tity of cadmium oxide a major portion ofwhich is in the form of relatively large particles, and

another of said materials consisting of a stillsmaller quantity of ametallic salt of the group consisting of silver iodide, silver bromide,silver chloride, cadmium chloride, and cadmium sulphate, said firstmentioned material being adapted to act as a matrix for said secondmentioned material, whereby the electrical conductivity of the former issubstantially unaffected by the presence of the latter, and said thirdmentioned material acting at the operating surface of the contact todecrease heating thereof due to arcing when in use and to preventbonding or sticking of such surface to a contact surface cooperatingtherewith.

18. A pressure molded and heat sintered electrical contact formed ofintimately mixed powdered materials, one of said materials consisting ofa relatively large quantity of metal of good electrical conductivity,another of said materials consisting of a relatively smaller quantity ofcadmium oxide a major portion of which is in the form of relativelylarge particles, and another of said materials consisting of a stillsmaller quantity of a metallic salt of the group consisting of silveriodide, silver bromide, silver chloride, cadmium chloride and cadmiumsulphate, said first mentioned material being adapted to. act as amatrix for said second mentioned material, whereby the electricalconductivity of the former is substantially unaffected by the presenceof the latter, and said second and third mentioned materials actingjointly to provide a film at the operating surface of the contact todecrease heating thereof due to arcing when in use and to preventbonding or sticking of such surface to the surface of a substantiallysimilar, contact cooperating therewith.

19. A pressure molded and heat sintered electrical contact composed ofintimately mixed powdered materials, one of said materials consisting offrom sixty to ninety-three per cent by weight of silver powder, anotherof said materials consisting of from seven to forty per cent of cadmiumoxide, and another of said materials consisting of from one-tenth of oneper cent to'ten per cent of a metallic salt of the group consisting ofsilver iodide, silver bromide, silver chloride, cadmium chloride andcadmium sulphate, a major portion of said cadmium oxide being in .heform of particles of a size greater than that which will pass through a300 mesh screen, said silver being adaptedtc act as a matrix for saidcadmium oxide, whereby the electrical conductivity of the former issubstantially unaffected by the presence of. the latter, andsaid cadmiumoxide and metallic salt acting jointly during use of the contact toprovide a film at the operating surface thereof, said film acting todecrease heating of such surface as an incident to arcing and to preventbonding or sticking of such surface to a contact surface cooperatingtherewith.

20. An electrical contact formed of powdered.

materials comprising-a metalof good electrical conductivity, a secondingredient consisting of cadmium oxide, and a metallic salt of the groupconsisting of silver iodide, silver bromide, silver chloride, cadmiumchloride, and cadmium sulphate, said second ingredient being present inan amount corresponding to seven to forty per cent of the entire mass,said metallic salt being present in an amount corresponding to one-tenthof one per cent to ten per cent of said mass, at least fifty per cent ofthe amount of said second ingredient consisting of particles of a sizegreater than that which will pass through a 300 mesh powdered silver,cadmium oxide and a silver halide, said cadmium oxide being present inan amount of not less than seven per cent nor more than forty per cent,and said silver halide being present in an amount of not less thanone-tenth of one per cent nor more than ten per cent.

22. An electrical contact consisting of fromseventy-flve per cent toeighty-seven and onehalf per cent by weight of metal of good electricalconductivity, and a relatively smaller proportion of cadmium oxide, themajor portion of said cadmium oxide being in theform of particles of asize greater than that which will pass through a 300 mesh screendispersed throughout the mass of said metal which serves as a matrixtherefor.

23. A pressure molded electrical contact consisting of a mixture of fromseventy-five per cent to eight-seven and one-half per cent by weight ofpowdered metal of good electrical conductivity, and a relatively smallerproportion of cadmium oxide densifled by heat sintering, the majorportion of said densified cadmium oxide being in the form of particlesof a size greater than that which will pass through a 300 mesh screendispersed throughout the'mass of said metal which serves as a matrixtherefor.

24. A pressure molded and heat sintered electrical contact comprising amixture of from sixty percent to ninety-three per cent by weight of finesilver powder, a relatively smaller proportion by weight of cadmiumoxide, and astill smaller proportion by weight of a metallic salt of thegroup consisting of silver iodide, silver bromide, silver chloride,cadmium chloride and cadmium sulphate, said cadmium oxide and saidmetallic salt being jointly in the form of powder densified by heatsintering, the major portion of said last mentioned powder being in theform of particles of a size greater than that which will pass through a300 mesh screen dispersed throughout the mass of said metal which servesas a matrix therefor.

25. The method of producing an electrical contact having improvednon-welding characteristics when in use, which consists in dispersingthroughout a quantity of metal of good electrical conductivity asubstantially smaller quantity of cadmium oxide,'th'e major portion ofsaid cadmium oxide being in the form of particles of relatively largesize, whereby said metal is adapted to act as a matrix for said cadmiumoxide without substantially affecting the electrical conductivity of theformer.

26. The method of producing an electrical contact having improvednon-welding characteristics when in use, which comprises intimatelymixing a relatively small quantity of a metallic salt of the groupconsisting of silver iodide, silver bromide, silver chloride, cadmiumchloride, and cadmium sulphate with a relatively larger quantity ofcadmium oxide, molding said mixture and sintering the same byapplication of heat to substantially increase the density thereof, thengrinding said molded mixture to form a granular powder the major portionof which consists of particles of a size greater than that which will:pass through a 300 mesh screen, intimately mixing said powder with asubstantially larger quantity of a powdered metal of good electricalconductivity, and then molding said last 27. The method of producing anelectrical contact having improved non-welding characteristics when inuse, which comprises intimately mixing a relatively small quantity of ametallic salt of the group consisting of silver iodide, silver bromide,silver chloride, cadmium chloride, and

cadmium sulphate with a relatively larger quantity of cadmium oxide,sintering said mixture by application of heat to substantially increasethe density thereof, then grinding said mixture to form a granularpowder the major portion of which consists of particles of a sizegreater than that which will pass through a 300 mesh screen,

intimately mixing said powder with a substan-.

tially larger quantity of a powdered metal of good electricalconductivity, then molding said last mentioned mixture under highpressure to provide an article of a desired shape, and then subjectingsaid article to a sintering heat.

28. The method of producing an electrical contact having improvednon-welding characteristics when in use, which comprises intimatelymixing a relatively small quantity of a metallic salt of the groupconsisting of silver iodide, silver bromide, silver chloride, cadmiumchloride, and cadmium sulphate with a relatively larger quantity ofcadmium oxide, sintering said mixture by ap 'ication of heat tosubstantially increase the de .ty thereof, then grinding said mixture toform a granular powder the major portion of which consists of particlesof a size greater than that which will pass through a 300 mesh screen,intimately mixing said powder with a substantially larger quantity of apowdered metal of good electrical conductivity, then molding said lastmentioned mixture under high pressure to provide an article of a desiredshape, then subjecting said article to a sintering heat, and thereaftersubjecting said article to a higher molding pressure to provide thefinished article.

29, The method of producing an electrical contact having improvednon-welding characteristics when in use, which comprises intimatelymixing a relatively small quantity of a silver halide with a relativelylarger quantity of cadmium oxide, sintering said mixture by applicationof heat to substantially increase the density thereof,

10 then grinding said mixture to form a granular powder adapted to passthrough a mesh screen, the major portion of said powder consisting ofparticles of a size greater than that which will pass through a 300 meshscreen, intimately mixing said powder with a substantially largerquantity of precipitated silver powder of fibrous texture, then formingsaid last mentioned mixture under a pressure of approximately fortythousand pounds per square inch in a mechanical molding press into anarticle of a desired shape, and then subjecting the molded article to aheat treatment for sintering thereof.

30. The method of producing an electrical contact having improvednon-welding characteristics when in use, which comprises intimatelymixing a relatively small quantity of a silver halide with a relativelylarger quantity of cadmium oxide, sintering said mixture by applicationof heat to substantially increase the density thereof, then grindingsaid mixture to form a granular powder adapted to pass through a 100mesh screen, the major portion of said powder consisting of particles ofa size greater than that which will pass through a 300 mesh screen,intimately mixing said powder with a substantially larger quantity ofprecipitated silver powder, then forming said last mentioned mixtureunder a pressure of approximately forty thousand pounds per square inchin a mechanical molding press into an article of a desired shape, thensubjecting the molded article to a heat treatment for sintering thereof,and thereafter subjecting said article to a molding pressure ofapproximately one hundred thousand pounds per square inch to produce afinished contact.

MERLE R. SWINEHART.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS

